Fish disease and injuries pose serious problems in the cultivation of various species of fish. Fish disease can spread to aquaculture, which is the husbandry of aquatic animals and plants, and cause to kill all the fish in aquaculture, which results in devastating damage to aquaculture. Contamination of infectious agents in aquaculture make it impossible to culture fish in the same facilities again. These problems have been a major problem of stable supply of fish. In addition, to prevent and treat fish disease, a large amount of antibiotics has been used for culturing fish, and the use of a large amount of antibiotics may cause problems in human health. To prevent or solve these problems, the present inventors have contemplated a method for enhancing a self defense system of fish by externally administering immunologically active substances to fish. These substances are originally secreted out from the leukocytes of fish so that it is necessary to isolate such cells from fish to establish a cell line. The cell line should be capable of indefinite growth and of producing immunologically active substances.
There have been various methods of obtaining cell lines capable of indefinite growth. These methods use hybridomas that tumor cells fused with lympocytes obtained from human (Kohler, G. et al., 1975, Nature 256: 495), transformants that are produced by transforming cells with tumor causing herpesvirus or EBV (Epstein, M. A., & Achong, B. G., 1979, The Epstein-Barr Virus Springer-Verglag, Berlin), T lympocytes transformed with HTLV-1 (Sugamura, K., et al., 1984, J. Immunol. Methods, 34: 221) and pre-B cells transformed with Ab-MLV (Reth, M. G., et al., 1985, 317:353). Cells derived from B lymphocytes are useful for producing hybridomas while T lymphocytes and leukocytes are not suitable for producing hybridomas. In fact, there has been no report on hybridomas produced from these cells so far. Serious drawbacks of hybridomas produced by cell fusion are their instability and undesirable changes in their properties during culture and subculture. Transformation is useful for producing transformants from leukocytes using various viruses. The transformants, however, also have properties of viruses so that the properties of the transformants are totally different from those of the original cells.